One image-forming apparatus well known in the art is a color electrophotographic printer employing a horizontal tandem system in which photosensitive drums are juxtaposed horizontally. Systems for forming images with color electrophotographic printers include an intermediate transfer system and a direct transfer system.
A color electrophotographic printer employing an intermediate transfer system has four photosensitive drums; and a charger, exposure unit, developing unit, transfer unit, and the like disposed around each photosensitive drum. This type of printer performs image formation as follows. First, each charger applies a uniform charge to the surface of the respective photosensitive drum. Next, the exposure unit irradiates a light beam onto the surface of each photosensitive drum to form an electrostatic latent image thereon. Subsequently, each developing unit supplies toner to the surface of the respective photosensitive drum in order to develop the electrostatic latent image into a toner image. The toner image is transferred from each photosensitive drum onto an intermediate transfer belt of the transfer unit in a primary transfer. By performing the steps for each photosensitive drum, a toner image in each of the four colors is superimposed on the intermediate transfer belt, forming a composite image on the belt.
Next, the composite image is transferred onto a sheet of paper conveyed at a coordinated timing in a secondary transfer as the sheet passes between a secondary transfer roller of the transfer unit and the intermediate transfer belt.
Next, image formation will be described for a color electrophotographic printer employing a direct transfer system. With the direct transfer system, electrostatic latent images on the photosensitive drums are similarly developed with toner in each respective color, as described above. After performing the same steps for each color of toner, the toner images are sequentially transferred onto a sheet of paper carried on a conveying belt of the transfer unit at a coordinated timing as the sheet passes between each photosensitive drum and a corresponding transfer roller, thereby forming an image on the sheet of paper.
While image formation is not being performed, the color electrophotographic printer periodically tests whether the toner image formed on each photosensitive drum is superimposed at the correct position on the belt. In order to detect the superimposing positions with accuracy, these tests are performed using a sensor disposed at a position opposing a roller on which the conveying belt is mounted, that is, in a region of the belt having no slack. A toner image formed as a patch pattern on each photosensitive drum is transferred onto the intermediate transfer belt or the paper-conveying belt and is conveyed to the sensor position. The sensor optically scans the patch patterns to detect problems in color registration and the like.
The patch patterns are subsequently recovered by a belt cleaner disposed in confrontation with the belt farther downstream of the sensor and on the side of the belt opposite the photosensitive drums (see Japanese unexamined patent application publication No. 2007-292789, for example). Hence, conventional means for recovering toner from a conveying belt is disposed on the opposite side of the belt from the photosensitive drums.
Generally, after toner images are transferred from photosensitive drums onto an intermediate transfer belt or paper, a drum cleaner recovers residual toner remaining on the photosensitive drum after a transfer operation.
However, one printer structure known in the art does not provide separate photosensitive drum cleaners, but uses the belt cleaner serving to recover residual toner from the belt also as a means to recover residual toner from the photosensitive drums.
For example, Japanese unexamined patent application publication No. 2000-29365 describes a printer including retaining means that temporarily retains residual toner collected from a photosensitive drum while images are being formed on the photosensitive drum (this period is hereinafter referred to as “during an image-forming operation”), and a belt cleaner for recovering residual toner from the belt. Residual toner temporarily retained by the retaining means is subsequently transferred to the belt and collected by the belt cleaner.